Oil and gas wells commonly bypass significant productive formations that may be uneconomic to complete at the time the wells were drilled. These formations may be relatively thin and low pressure so simply perforating a zone that includes oil does not provide significant new production. Lateral drilling tools have been developed that are capable of drilling formations using rotary mechanical or jetting tools. Lateral drilling into thin, horizontal oil bearing formations can result in substantial new oil production. The lateral well must be drilled at an angle as close as possible to 90 degrees to ensure that the lateral drilling tools stay within the productive zone and can be achieved by feeding a flexible lance though a shoe that curves to form a right angle, directing the lance into the formation. This approach is referred to as zero radius lateral drilling, since the angle is built entirely within the casing as opposed to being formed by drilling a curved hole in the formation.
In the event that the well is cased, lateral drilling requires milling a window in the steel casing before the lateral drilling tool is introduced. Zero radius lateral drilling requires milling a circular or slightly elliptical window in the casing. The milling assembly is preferably directed toward the casing through the same curved shoe that will be used to direct the lateral drilling lance. The shoe incorporates a tight radius curve, providing a near 90 degree turn within the inner diameter (ID) of the casing. The shoe can be set using conventional mechanical or hydraulic packers to ensure that a stable hole location for the jetting assembly is achieved, once the milling is completed.
Milling the steel casing requires substantial torque at relatively low rotary speed. The tool can be rotated by using a rotary table and drillstring, or by using a downhole motor. The thrust, torque, and rotary motion must be transmitted though a flexible assembly that will pass though the shoe. A number of approaches have been developed to achieve this goal; however, all have met with substantial practical difficulties.
It would thus be desirable to provide a method and apparatus for milling such a lateral window in a drill casing that avoids the problems experienced in the earlier attempted approaches.